The present invention is generally directed to a method for making multiphase composite materials directly from solution precursor droplets by a fast pyrolysis process using microwave generated plasma.
In recent years, the advent of multiphase nanostructure composites of metal oxide ceramics has undergone a leap in interest as a natural improvement of coarse grain or even single phase nanostructures of these materials. It was found that mechanical, thermal, optical, chemical, electrical and magnetic material properties can be drastically improved as the grain size is reduced from the coarse scale in micrometers to a nanometer scale, typically with grain size below 100 nanometers (nm). Furthermore, these nanocomposite materials exhibit a much stable phase than their counterpart, single phase materials. The presence of several phases in one matrix tends to inhibit grain growth during thermal heating. The properties of these new materials are also influenced by the nanoscale grain boundaries prone to site pinning and responsible for phase micro structure stability. Another stringent condition to achieve phase stability is the production of these multiphase nanocomposites with a fine and uniform distribution of phase domains in the nanocomposite matrix.
Many synthetic methods have been used to synthesize these nanocomposite materials to control micro structure length scales and the distribution of the elements in the composition. Most methods are unable to achieve both conditions due to the complexity of chemical, thermal, and nucleation rates of the matrix components, with the added difficulty of the physical and chemical properties of the solvents involved. Some can achieve both but they require the use of several thermal processing steps to achieve nanoscale grains and phase homogeneity of the constituents matrix. Jordan et al. (US Patent Application # US20120322645, 2012) used a sol-gel esterification technique to produce magnesia-yttrium particles suitable for infra-red window application. This invention uses three main steps: step 1 consists of moderate heating at low temperature to evaporate water and form a foam consisting of the complexion network of organic acid and alcohol necessary to achieve the homogenous dispersion of metal oxide cations; step 2 consists of thermal heating up to 400° C. to eliminate all carbon embedded in the foam while keeping grain size below 20 nanometers (nm); step 3 uses thermal treatment up to 1100° C. to achieve full crystallinity of the magnesia-yttrium nanocomposite with grain size about 100 nm. Major drawbacks of such approach include the fact that it is not easily scalable, as it will require large furnaces, and requires hours, if not days, of thermal heating to eliminate the solvents, and also achieve full crystallization of the final product.
A method that achieves ultrafine and somewhat homogenous metal oxide nanocomposites is Liquid-Feed-Flame-Pyrolysis by R. Laine et al. (U.S. Pat. No. 7,700,152, 2010). This method injects atomized droplets of metal precursors into a combustion flame to produce nanocomposite particles powders in few milliseconds, similar to the present invention. However, this method suffers from some drawbacks including non-uniform size and size distribution of particles due to atomization, and non homogenous thermal heating of droplets due to large temperature gradient across the flame whose temperature does not exceed 2000° C. This results in non-homogeneity of phase micro structure of composition distribution in the final product. Post processing steps involving cyclones and ceramic filters are required to separate large agglomerates from nanoscale particles.
Another method that features the 1-step approach for the production of nanocomposite materials uses radio frequency plasma to process atomized droplets of metal precursors (Boulos, U.S. Pat. No. 6,919,527 B2, 2005). Although high temperature and axisymmetry of physical embodiment to contain the plasma are achieved, this method still suffers from non uniformity of composition due to, in part, injection of atomized liquid precursors of variable sizes, but also to the non uniformity of the plasma which exhibits a hollow core due to skin effect. Particles passing through the core of the plasma tend not to be fully processed compared to the particles passing through the peripheral part of the plasma. This leads to non homogeneity of particle processing and production of particles with homogeneous phase microstructure.
From the above, it is therefore seen that there exists a need in the art to overcome the deficiencies and limitations described herein and above.